I have a Mauser with 26" Douglass Premium barrell in .458 WM.
Only for example of velocity with this long barrell, this is my own reloads with Hornady 500 FMJ:
WW cases, Fed 210, IMR 3031 # 72.5 gr. 2.221 fts.
WW cases, Fed 210, IMR 4895 # 72 gr. 2.182 fts.
WW cases, Fed 215, WW 748 # 79 gr. 2.057 fts.
Hector